1. Field of the Invention
This invention relates to electric devices and more particularly to functional electric devices such as heat-sensitive, fusing, photovaltaic and the like devices.
2. Description of the Prior Art
In general, functional electric devices are fundamentally comprised of a functional material layer and at least a pair of electrodes which contact with the functional material layer and are spaced from each other. Upon application of AC or DC voltage to the layer through the electric conductors, it is possible to accurately detect a resistance, electrostatic capacitance or impedance which depends on the volume resistitivity or dielectric constant inherent to the material used, or a variation in these electrical characteristics. By the term "functional material" used herein is meant a material whose electrical characteristic such as resistance, electrostatic capacitance or impedance or its variation can be measured and utilized for some intended purposes. A variety of these functional materials are known, including materials such as ceramics, polymeric materials, organic materials such as organic semiconductors, dyes, etc., and the like. Among them, polymeric functional materials have now been widely used in the field of sensors for detecting environmental variations. For instance, there are known piezoelectric elements using polyfluorovinylidene compositions, heat-sensitive elements making use of polyamide compositions or compositions obtined by dispersing organic semiconductors in polyolefin resins such as polyethylene or polypropylene.
These elements are constructed, for example, such that one electrically conductive wire is spirally wound about a core and is then convered on the outer surface thereof with a functional polymer layer, which is in turn spirally wound with another elecrtrical conductive wire and the entirety is enclosed in an insulating cover to give a coaxial wire which is flexible as a whole. Alternatively, the element may have such a construction that a sheet-like functional polymer layer is formed and attached with electrical conductive wires on opposite surfaces thereof, and is accommodated in an insulating enclosure to give a multilayer plate. Of these, the coaxial wire has wide utility as a heatsensitive element of heating apparatus such as electric blankets.
In these functional electric elements, it is common to use copper or its alloys as a material for the electrically conductive wire or electrode. This is because copper has good conductivity and is excellent in workability. In this connection, however, copper is apt to undergo corrosion or other deterioration upon contact with an environmental atmosphere or functional material layer. Especially when copper wire undergoes such a change as mentioned above at portions in contacting a functional material, electrical characteristics such as impedance or other properties of the functional material cannot be accurately detected. When polymeric or organic functional materials which contain plasticizers, stabilizers, ionic substances or other impurities are used, copper is readily attacked by these substances and at the same time the layer of the polymer or organic material is adversely influenced or even decomposed by the attacked copper. These phenomena have considerably limited the application for copper or copper alloys as electrodes in functional electric elements. In addition, the phonomena become more pronounced when functional electric elements are employed under high temperature or high humidity conditions. This is because under these conditions, electrical contact between the functional material layer and the copper or copper alloy electrodes is impeded, making it almost impossible to accurately detect an electrical characteristic such as resistance, electrostatic capacitance or impedance or other changes in the functional material layer. In addition, since the functional polymer layer itself is attacked by copper and undergoes a change in quality, the above defect becomes more pronounced.